We wish to design, construct and operate a single sector at the Advanced Photon Source (APS) under construction at Argonne National Laboratory. The sector will consist of an undulator and wiggler source, a bending magnet source, associated x-ray optical elements, beam transport, three hutches, a BL3 biohazards facility, and experimental apparatus suitable for five different styles of experiments in structural biology: virus crystallography, small angle scattering, protein crystallography/multiple wavelength anomalous dispersion, time-resolved diffraction, and diffraction from microcrystals. The sector will be operated for the 29-member group of structural biologists denoted BioCARS (roughly 75% of the available beamtime) and for general users (roughly 25%). Technological research and development will concentrate on: X-ray optics, including transmission optics and cooling schemes that will enable the optics to withstand the high heat loads imposed by the insertion device sources without significant degradation of their properties; storage phosphor and other detectors; sample handling devices such as liquid helium cryostats and thin film crystal mounts; software for efficient, user-friendly data acquisition, reduction and storage of both static and time-resolved data; and, novel experimental techniques. Collaborative and service projects will encompass X-ray diffraction from single crystals and from less-ordered membranes, fibers and solutions. Particular emphasis will be placed on crystals with very large unit cells such as viruses, and on time-resolved studies on time scales down to and including the x-ray pulse length at the APS, around 100 ps. These studies exploit the unique properties of the APS: its brilliance, intensity, and dedicated availability. Prior to the start-up of the APS in 1995-96, initial experiments and tests of X-ray equipment will be conducted at the National Synchrotron Light Source, on beam line X26C. BioCARS forms an integral part of the interdisciplinary Consortium for Advanced Radiation Sources, CARS. CARS consist of three universities (The University of Chicago, Northern Illinois University, and Southern Illinois University) and four national scientific groups, in structural biology (BioCARS), the earth and planetary sciences (GeoCARS), chemical sciences (ChemCARS) and soil and environmental science (Soil/EnviroCARS). CARS at present consists of 97 principal investigators, and is a Collaborative Access Team (CAT) which proposes to develop and operate two sectors at the APS, of which this request is for support of one. Substantial facilities, obtained with State and University support, are already in place or committed, including design, construction, X-ray, and metrology laboratories.